January 1980

Transmission Control Protocol

January 1980

Transmission Control Protocol

PREFACE

This document describes the DoD Standard Transmission Control Protocol
(TCP). There have been eight earlier editions of the ARPA TCP
specification on which this standard is based, and the present text
draws heavily from them. There have been many contributors to this work
both in terms of concepts and in terms of text. This edition
incorporates the addition of security, compartmentation, and precedence
concepts into the TCP specification.

DOD STANDARD

TRANSMISSION CONTROL PROTOCOL

1. INTRODUCTION

The Transmission Control Protocol (TCP) is intended for use as a highly
reliable host-to-host protocol between hosts in packet-switched computer
communication networks, and especially in interconnected systems of such
networks.

This document describes the functions to be performed by the
Transmission Control Protocol, the program that implements it, and its
interface to programs or users that require its services.

1.1. Motivation

Computer communication systems are playing an increasingly important
role in military, government, and civilian environments. This
document primarily focuses its attention on military computer
communication requirements, especially robustness in the presence of
communication unreliability and availability in the presence of
congestion, but many of these problems are found in the civilian and
government sector as well.

As strategic and tactical computer communication networks are
developed and deployed, it is essential to provide means of
interconnecting them and to provide standard interprocess
communication protocols which can support a broad range of
applications. In anticipation of the need for such standards, the
Deputy Undersecretary of Defense for Research and Engineering has
declared the Transmission Control Protocol (TCP) described herein to
be a basis for DoD-wide inter-process communication protocol
standardization.

TCP is a connection-oriented, end-to-end reliable protocol designed to
fit into a layered hierarchy of protocols which support multi-network
applications. The TCP provides for reliable inter-process
communication between pairs of processes in host computers attached to
distinct but interconnected computer communication networks. Very few
assumptions are made as to the reliability of the communication
protocols below the TCP layer. TCP assumes it can obtain a simple,
potentially unreliable datagram service from the lower level
protocols. In principle, the TCP should be able to operate above a
wide spectrum of communication systems ranging from hard-wired
connections to packet-switched or circuit-switched networks.

January 1980
Transmission Control Protocol
Introduction

TCP is based on concepts first described by Cerf and Kahn in [1]. The
TCP fits into a layered protocol architecture just above a basic
Internet Protocol [2] which provides a way for the TCP to send and
receive variable-length segments of information enclosed in internet
datagram "envelopes". The internet datagram provides a means for
addressing source and destination TCPs in different networks. The
internet protocol also deals with any fragmentation or reassembly of
the TCP segments required to achieve transport and delivery through
multiple networks and interconnecting gateways. The internet protocol
also carries information on the precedence, security classification
and compartmentation of the TCP segments, so this information can be
communicated end-to-end across multiple networks.

Protocol Layering

Figure 1

Much of this document is written in the context of TCP implementations
which are co-resident with higher level protocols in the host
computer. As a practical matter, many computer systems will be
connected to networks via front-end computers which house the TCP and
internet protocol layers, as well as network specific software. The
TCP specification describes an interface to the higher level protocols
which appears to be implementable even for the front-end case, as long
as a suitable host-to-front end protocol is implemented.

1.2. Scope

The TCP is intended to provide a reliable process-to-process
communication service in a multinetwork environment. The TCP is
intended to be a host-to-host protocol in common use in multiple
networks.

1.3. About this Document

This document represents a specification of the behavior required of
any TCP implementation, both in its interactions with higher level
protocols and in its interactions with other TCPs. The rest of this

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Transmission Control Protocol
Introduction

section offers a very brief view of the protocol interfaces and
operation. Section 2 summarizes the philosophical basis for the TCP
design. Section 3 offers both a detailed description of the actions
required of TCP when various events occur (arrival of new segments,
user calls, errors, etc.) and the details of the formats of TCP
segments.

1.4. Interfaces

The TCP interfaces on one side to user or application processes and on
the other side to a lower level protocol such as Internet Protocol.

The interface between an application process and the TCP is
illustrated in reasonable detail. This interface consists of a set of
calls much like the calls an operating system provides to an
application process for manipulating files. For example, there are
calls to open and close connections and to send and receive letters on
established connections. It is also expected that the TCP can
asynchronously communicate with application programs. Although
considerable freedom is permitted to TCP implementors to design
interfaces which are appropriate to a particular operating system
environment, a minimum functionality is required at the TCP/user
interface for any valid implementation.

The interface between TCP and lower level protocol is essentially
unspecified except that it is assumed there is a mechanism whereby the
two levels can asynchronously pass information to each other.
Typically, one expects the lower level protocol to specify this
interface. TCP is designed to work in a very general environment of
interconnected networks. The lower level protocol which is assumed
throughout this document is the Internet Protocol [2].

1.5. Operation

As noted above, the primary purpose of the TCP is to provide reliable,
securable logical circuit or connection service between pairs of
processes. To provide this service on top of a less reliable internet
communication system requires facilities in the following areas:

The basic operation of the TCP in each of these areas is described in
the following paragraphs.

January 1980
Transmission Control Protocol
Introduction

Basic Data Transfer:

The TCP is able to transfer a continuous stream of octets in each
direction between its users by packaging some number of octets into
segments for transmission through the internet system. In this
stream mode, the TCPs decide when to block and forward data at their
own convenience.

For users who desire a record-oriented service, the TCP also permits
the user to submit records, called letters, for transmission. When
the sending user indicates a record boundary (end-of-letter), this
causes the TCPs to promptly forward and deliver data up to that
point to the receiver.

Reliability:

The TCP must recover from data that is damaged, lost, duplicated, or
delivered out of order by the internet communication system. This
is achieved by assigning a sequence number to each octet
transmitted, and requiring a positive acknowledgment (ACK) from the
receiving TCP. If the ACK is not received within a timeout
interval, the data is retransmitted. At the receiver, the sequence
numbers are used to correctly order segments that may be received
out of order and to eliminate duplicates. Damage is handled by
adding a checksum to each segment transmitted, checking it at the
receiver, and discarding damaged segments.

As long as the TCPs continue to function properly and the internet
system does not become completely partitioned, no transmission
errors will affect the users. TCP recovers from internet
communication system errors.

Flow Control:

TCP provides a means for the receiver to govern the amount of data
sent by the sender. This is achieved by returning a "window" with
every ACK indicating a range of acceptable sequence numbers beyond
the last segment successfully received. For stream mode, the window
indicates an allowed number of octets that the sender may transmit
before receiving further permission. For record mode, the window
indicates an allowed amount of buffer space the sender may consume,
this may be more than the number of data octets transmitted if there
is a mismatch between letter size and buffer size.

January 1980
Transmission Control Protocol
Introduction

Multiplexing:

To allow for many processes within a single Host to use TCP
communication facilities simultaneously, the TCP provides a set of
addresses or ports within each host. Concatenated with the network
and host addresses from the internet communication layer, this forms
a socket. A pair of sockets uniquely identifies each connection.
That is, a socket may be simultaneously used in multiple
connections.

The binding of ports to processes is handled independently by each
Host. However, it proves useful to attach frequently used processes
(e.g., a "logger" or timesharing service) to fixed sockets which are
made known to the public. These services can then be accessed
through the known addresses. Establishing and learning the port
addresses of other processes may involve more dynamic mechanisms.

Connections:

The reliability and flow control mechanisms described above require
that TCPs initialize and maintain certain status information for
each data stream. The combination of this information, including
sockets, sequence numbers, and window sizes, is called a connection.
Each connection is uniquely specified by a pair of sockets
identifying its two sides.

When two processes wish to communicate, their TCP's must first
establish a connection (initialize the status information on each
side). When their communication is complete, the connection is
terminated or closed to free the resources for other uses.

Since connections must be established between unreliable hosts and
over the unreliable internet communication system, a handshake
mechanism with clock-based sequence numbers is used to avoid
erroneous initialization of connections.

Precedence and Security:

The users of TCP may indicate the security and precedence of their
communication. Provision is made for default values to be used when
these features are not needed.

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Transmission Control Protocol

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Transmission Control Protocol

2. PHILOSOPHY

2.1. Elements of the Internetwork System

The internetwork environment consists of hosts connected to networks
which are in turn interconnected via gateways. It is assumed here
that the networks may be either local networks (e.g., the ETHERNET) or
large networks (e.g., the ARPANET), but in any case are based on
packet switching technology. The active agents that produce and
consume messages are processes. Various levels of protocols in the
networks, the gateways, and the hosts support an interprocess
communication system that provides two-way data flow on logical
connections between process ports.

We specifically assume that data is transmitted from host to host
through means of a set of networks. When we say network, we have in
mind a packet switched network (PSN). This assumption is probably
unnecessary, since a circuit switched network or a hybrid combination
of the two could also be used; but for concreteness, we explicitly
assume that the hosts are connected to one or more packet switches of
a PSN.

The term packet is used generically here to mean the data of one
transaction between a host and a packet switch. The format of data
blocks exchanged between the packet switches in a network will
generally not be of concern to us.

Hosts are computers attached to a network, and from the communication
network's point of view, are the sources and destinations of packets.
Processes are viewed as the active elements in host computers (in
accordance with the fairly common definition of a process as a program
in execution). Even terminals and files or other I/O devices are
viewed as communicating with each other through the use of processes.
Thus, all communication is viewed as inter-process communication.

Since a process may need to distinguish among several communication
streams between itself and another process (or processes), we imagine
that each process may have a number of ports through which it
communicates with the ports of other processes.

2.2. Model of Operation

Processes transmit data by calling on the TCP and passing buffers of
data as arguments. The TCP packages the data from these buffers into
segments and calls on the internet module to transmit each segment to
the destination TCP. The receiving TCP places the data from a segment
into the receiving user's buffer and notifies the receiving user. The
TCPs include control information in the segments which they use to
ensure reliable ordered data transmission.

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Transmission Control Protocol
Philosophy

The model of internet communication is that there is an internet
protocol module associated with each TCP which provides an interface
to the local network. This internet module packages TCP segments
inside internet datagrams and routes these datagrams to a destination
internet module or intermediate gateway. To transmit the datagram
through the local network, it is embedded in a local network packet.

The packet switches may perform further packaging, fragmentation, or
other operations to achieve the delivery of the local packet to the
destination internet module.

At a gateway between networks, the internet datagram is "unwrapped"
from its local packet and examined to determine through which network
the internet datagram should travel next. The internet datagram is
then "wrapped" in a local packet suitable to the next network and
routed to the next gateway, or to the final destination.

A gateway is permitted to break up an internet datagram into smaller
internet datagram fragments if this is necessary for transmission
through the next network. To do this, the gateway produces a set of
internet datagrams; each carrying a fragment. Fragments may be broken
into smaller ones at intermediate gateways. The internet datagram
fragment format is designed so that the destination internet module
can reassemble fragments into internet datagrams.

A destination internet module unwraps the segment from the datagram
(after reassembling the datagram, if necessary) and passes it to the
destination TCP.

This simple model of the operation glosses over many details. One
important feature is the type of service. This provides information
to the gateway (or internet module) to guide it in selecting the
service parameters to be used in traversing the next network.
Included in the type of service information is the precedence of the
datagram. Datagrams may also carry security information to permit
host and gateways that operate in multilevel secure environments to
properly segregate datagrams for security considerations.

2.3. The Host Environment

The TCP is assumed to be a module in a time sharing operating system.
The users access the TCP much like they would access the file system.
The TCP may call on other operating system functions, for example, to
manage data structures. The actual interface to the network is
assumed to be controlled by a device driver module. The TCP does not
call on the network device driver directly, but rather calls on the
internet datagram protocol module which may in turn call on the device
driver.

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Transmission Control Protocol
Philosophy

Though it is assumed here that processes are supported by the host
operating system, the mechanisms of TCP do not preclude implementation
of the TCP in a front-end processor. However, in such an
implementation, a host-to-front-end protocol must provide the
functionality to support the type of TCP-user interface described
above.

2.4. Interfaces

The TCP/user interface provides for calls made by the user on the TCP
to OPEN or CLOSE a connection, to SEND or RECEIVE data, or to obtain
STATUS about a connection. These calls are like other calls from user
programs on the operating system, for example, the calls to open, read
from, and close a file.

The TCP/internet interface provides calls to send and receive
datagrams addressed to TCP modules in hosts anywhere in the internet
system. These calls have parameters for passing the address, type of
service, precedence, security, and other control information.

2.5. Relation to Other Protocols

The following diagram illustrates the place of the TCP in the protocol
hierarchy:

Protocol Relationships

Figure 2.

It is expected that the TCP will be able to support higher level
protocols efficiently. It should be easy to interface higher level
protocols like the ARPANET Telnet [3] or AUTODIN II THP to the TCP.

2.6. Reliable Communication

A stream of data sent on a TCP connection is delivered reliably and in
order at the destination.

Transmission is made reliable via the use of sequence numbers and
acknowledgments. Conceptually, each octet of data is assigned a
sequence number. The sequence number of the first octet of data in a
segment is the sequence number transmitted with that segment and is
called the segment sequence number. Segments also carry an
acknowledgment number which is the sequence number of the next
expected data octet of transmissions in the reverse direction. When
the TCP transmits a segment, it puts a copy on a retransmission queue
and starts a timer; when the acknowledgment for that data is received,
the segment is deleted from the queue. If the acknowledgment is not
received before the timer runs out, the segment is retransmitted.

An acknowledgment by TCP does not guarantee that the data has been
delivered to the end user, but only that the receiving TCP has taken
the responsibility to do so.

To govern the flow of data into a TCP, a flow control mechanism is
employed. The the data receiving TCP reports a window to the sending
TCP. This window specifies the number of octets, starting with the
acknowledgment number that the data receiving TCP is currently
prepared to receive.

2.7. Connection Establishment and Clearing

To identify the separate data streams that a TCP may handle, the TCP
provides a port identifier. Since port identifiers are selected
independently by each operating system, TCP, or user, they might not
be unique. To provide for unique addresses at each TCP, we
concatenate an internet address identifying the TCP with a port
identifier to create a socket which will be unique throughout all
networks connected together.

A connection is fully specified by the pair of sockets at the ends. A
local socket may participate in many connections to different foreign
sockets. A connection can be used to carry data in both directions,
that is, it is "full duplex".

TCPs are free to associate ports with processes however they choose.
However, several basic concepts seem necessary in any implementation.

January 1980
Transmission Control Protocol
Philosophy

There must be well-known sockets which the TCP associates only with
the "appropriate" processes by some means. We envision that processes
may "own" ports, and that processes can only initiate connections on
the ports they own. (Means for implementing ownership is a local
issue, but we envision a Request Port user command, or a method of
uniquely allocating a group of ports to a given process, e.g., by
associating the high order bits of a port name with a given process.)

A connection is specified in the OPEN call by the local port and
foreign socket arguments. In return, the TCP supplies a (short) local
connection name by which the user refers to the connection in
subsequent calls. There are several things that must be remembered
about a connection. To store this information we imagine that there
is a data structure called a Transmission Control Block (TCB). One
implementation strategy would have the local connection name be a
pointer to the TCB for this connection. The OPEN call also specifies
whether the connection establishment is to be actively pursued, or to
be passively waited for.

A passive OPEN request means that the process wants to accept incoming
connection requests rather than attempting to initiate a connection.
Often the process requesting a passive OPEN will accept a connection
request from any caller. In this case a foreign socket of all zeros
is used to denote an unspecified socket. Unspecified foreign sockets
are allowed only on passive OPENs.

A service process that wished to provide services for unknown other
processes could issue a passive OPEN request with an unspecified
foreign socket. Then a connection could be made with any process that
requested a connection to this local socket. It would help if this
local socket were known to be associated with this service.

Well-known sockets are a convenient mechanism for a priori associating
a socket address with a standard service. For instance, the
"Telnet-Server" process might be permanently assigned to a particular
socket, and other sockets might be reserved for File Transfer, Remote
Job Entry, Text Generator, Echoer, and Sink processes (the last three
being for test purposes). A socket address might be reserved for
access to a "Look-Up" service which would return the specific socket
at which a newly created service would be provided. The concept of a
well-known socket is part of the TCP specification, but the assignment
of sockets to services is outside this specification.

Processes can issue passive OPENs and wait for matching calls from
other processes and be informed by the TCP when connections have been
established. Two processes which issue calls to each other at the
same time are correctly connected. This flexibility is critical for

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Transmission Control Protocol
Philosophy

the support of distributed computing in which components act
asynchronously with respect to each other.

There are two cases for matching the sockets in the local request and
an incoming segment. In the first case, the local request has fully
specified the foreign socket. In this case, the match must be exact.
In the second case, the local request has left the foreign socket
unspecified. In this case, any foreign socket is acceptable as long
as the local sockets match.

If there are several pending passive OPENs (recorded in TCBs) with the
same local socket, an incoming segment should be matched to a request
with the specific foreign socket in the segment, if such a request
exists, before selecting a request with an unspecified foreign socket.

The procedures to establish and clear connections utilize synchronize
(SYN) and finis (FIN) control flags and involve an exchange of three
messages. This exchange has been termed a three-way hand shake [4].

A connection is initiated by the rendezvous of an arriving segment
containing a SYN and a waiting TCB entry created by a user OPEN
command. The matching of local and foreign sockets determines when a
connection has been initiated. The connection becomes "established"
when sequence numbers have been synchronized in both directions.

The clearing of a connection also involves the exchange of segments,
in this case carrying the FIN control flag.

2.8. Data Communication

The data that flows on a connection may be thought of as a stream of
octets, or as a sequence of records. In TCP the records are called
letters and are of variable length. The sending user indicates in
each SEND call whether the data in that call completes a letter by the
setting of the end-of-letter parameter.

The length of a letter may be such that it must be broken into
segments before it can be transmitted to its destination. We assume
that the segments will normally be reassembled into a letter before
being passed to the receiving process. A segment may contain all or a
part of a letter, but a segment never contains parts of more than one
letter. The end of a letter is marked by the appearance of an EOL
control flag in a segment. A sending TCP is allowed to collect data
from the sending user and to send that data in segments at its own
convenience, until the end of letter is signaled then it must send all
unsent data. When a receiving TCP has a complete letter, it must not
wait for more data from the sending TCP before passing the letter to
the receiving process.

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Transmission Control Protocol
Philosophy

There is a coupling between letters as sent and the use of buffers of
data that cross the TCP/user interface. Each time an end-of-letter
(EOL) flag is associated with data placed into the receiving user's
buffer, the buffer is returned to the user for processing even if the
buffer is not filled. If a letter is longer than the user's buffer,
the letter is passed to the user in buffer size units, the last of
which may be only partly full. The receiving TCP's buffer size may be
communicated to the sending TCP when the connection is being
established.

The TCP is responsible for regulating the flow of segments on the
connections, as a way of preventing itself from becoming saturated or
overloaded with traffic. This is done using a window flow control
mechanism. The data receiving TCP reports to the data sending TCP a
window which is the range of sequence numbers of data octets that data
receiving TCP is currently prepared to accept.

TCP also provides a means to communicate to the receiver of data that
at some point further along in the data stream than the receiver is
currently reading there is urgent data. TCP does not attempt to
define what the user specifically does upon being notified of pending
urgent data, but the general notion is that the receiving process
should take action to read through the end urgent data quickly.

2.9. Precedence and Security

The TCP makes use of the internet protocol type of service field and
security option to provide precedence and security on a per connection
basis to TCP users. Not all TCP modules will necessarily function in
a multilevel secure environment, some may be limited to unclassified
use only, and others may operate at only one security level and
compartment. Consequently, some TCP implementations and services to
users may be limited to a subset of the multilevel secure case.

TCP modules which operate in a multilevel secure environment should
properly mark outgoing segments with the security, compartment, and
precedence. Such TCP modules should also provide to their users or
higher level protocols such as Telnet or THP an interface to allow
them to specify the desired security level, compartment, and
precedence of connections.

2.10. Robustness Principle

TCP implementations should follow a general principle of robustness:
be conservative in what you do, be liberal in what you accept from
others.

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Transmission Control Protocol

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Transmission Control Protocol

3. FUNCTIONAL SPECIFICATION

3.1. Header Format

TCP segments are sent as internet datagrams. The Internet Protocol
header carries several information fields, including the source and
destination host addresses [2]. A TCP header follows the internet
header, supplying information specific to the TCP protocol. This
division allows for the existence of host level protocols other than
TCP.

The number of data octets beginning with the one indicated in the
acknowledgment field which the sender of this segment is willing to
accept.

Checksum: 16 bits

The checksum field is the 16 bit one's complement of the one's
complement sum of all 16 bit words in the header and text. If a
segment contains an odd number of header and text octets to be
checksummed, the last octet is padded on the right with zeros to
form a 16 bit word for checksum purposes. The pad is not
transmitted as part of the segment. While computing the checksum,
the checksum field itself is replaced with zeros.

The checksum also covers a 96 bit pseudo header conceptually
prefixed to the TCP header. This pseudo header contains the Source

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Transmission Control Protocol
Functional Specification

Address, the Destination Address, the Protocol, and TCP length.
This gives the TCP protection against misrouted segments. This
information is carried in the Internet Protocol and is transferred
across the TCP/Network interface in the arguments or results of
calls by the TCP on the IP.

The TCP Length is the TCP header plus the data length in octets
(this is not an explicitly transmitted quantity, but is computed
from the total length, and the header length).

Urgent Pointer: 16 bits

This field communicates the current value of the urgent pointer as a
positive offset from the sequence number in this segment. The
urgent pointer points to the sequence number of the octet following
the urgent data. This field should only be interpreted in segments
with the URG control bit set.

Options: variable

Options may occupy space at the end of the TCP header and are a
multiple of 8 bits in length. All options are included in the
checksum. An option may begin on any octet boundary. There are two
cases for the format of an option:

Case 1: A single octet of option-kind.
Case 2: An octet of option-kind, an octet of option-length, and
the actual option-data octets.

The option-length counts the two octets of option-kind and
option-length as well as the option-data octets.

Note that the list of options may be shorter than the data offset
field might imply. The content of the header beyond the
End-of-Option option should be header padding (i.e., zero).

This option code indicates the end of the option list. This
might not coincide with the end of the TCP header according to
the Data Offset field. This is used at the end of all options,
not the end of each option, and need only be used if the end of
the options would not otherwise coincide with the end of the TCP
header.

No-Operation
+--------+
|00000001|
+--------+
Kind=1

This option code may be used between options, for example, to
align the beginning of a subsequent option on a word boundary.
There is no guarantee that senders will use this option, so
receivers must be prepared to process options even if they do
not begin on a word boundary.

If this option is present, then it communicates the receive
buffer size at the TCP which sends this segment. This field
should only be sent in the initial connection request (i.e.,
in segments with the SYN control bit set). If this option is
not used, the default buffer size of one octet is assumed.

Padding: variable

The TCP header padding is used to ensure that the TCP header ends
and data begins on a 32 bit boundary. The padding is composed of
zeros.

3.2. Terminology

Before we can discuss very much about the operation of the TCP we need
to introduce some detailed terminology. The maintenance of a TCP
connection requires the remembering of several variables. We conceive
of these variables being stored in a connection record called a
Transmission Control Block or TCB. Among the variables stored in the
TCB are the local and remote socket numbers, the security and
precedence of the connection, pointers to the user's send and receive
buffers, pointers to the retransmit queue and to the current segment.
In addition several variables relating to the send and receive
sequence numbers are stored in the TCB.

Send Sequence Space

Figure 4.

1 - old sequence numbers which have been acknowledged

2 - sequence numbers allowed for new reception

3 - future sequence numbers which are not yet allowed

Receive Sequence Space

Figure 5.

There are also some variables used frequently in the discussion that
take their values from the fields of the current segment.

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Functional Specification
Current Segment Variables
SEG.SEQ - segment sequence number
SEG.ACK - segment acknowledgment number
SEG.LEN - segment length
SEG.WND - segment window
SEG.UP - segment urgent pointer
SEG.PRC - segment precedence value
A connection progresses through a series of states during its
lifetime. The states are: LISTEN, SYN-SENT, SYN-RECEIVED,
ESTABLISHED, FIN-WAIT-1, FIN-WAIT-2, TIME-WAIT, CLOSE-WAIT, CLOSING,
and the fictional state CLOSED. CLOSED is fictional because it
represents the state when there is no TCB, and therefore, no
connection. Briefly the meanings of the states are:

LISTEN - represents waiting for a connection request from any remote
TCP and port.

SYN-SENT - represents waiting for a matching connection request
after having sent a connection request.

SYN-RECEIVED - represents waiting for a confirming connection
request acknowledgment after having both received and sent a
connection request.

ESTABLISHED - represents an open connection, ready to transmit and
receive data segments.

FIN-WAIT-1 - represents waiting for a connection termination request
from the remote TCP, or an acknowledgment of the connection
termination request previously sent.

FIN-WAIT-2 - represents waiting for a connection termination request
from the remote TCP.

TIME-WAIT - represents waiting for enough time to pass to be sure
the remote TCP received the acknowledgment of its connection
termination request.

CLOSE-WAIT - represents waiting for a connection termination request
from the local user.

A TCP connection progresses from one state to another in response to
events. The events are the user calls, OPEN, SEND, RECEIVE, CLOSE,
ABORT, and STATUS; the incoming segments, particularly those
containing the SYN and FIN flags; and timeouts.

The Glossary contains a more complete list of terms and their
definitions.

The state diagram in figure 6 only illustrates state changes, together
with the causing events and resulting actions, but addresses neither
error conditions nor actions which are not connected with state
changes. In a later section, more detail is offered with respect to
the reaction of the TCP to events.

TCP Connection State Diagram

Figure 6.

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Functional Specification

3.3. Sequence Numbers

A fundamental notion in the design is that every octet of data sent
over a TCP connection has a sequence number. Since every octet is
sequenced, each of them can be acknowledged. The acknowledgment
mechanism employed is cumulative so that an acknowledgment of sequence
number X indicates that all octets up to but not including X have been
received. This mechanism allows for straight-forward duplicate
detection in the presence of retransmission. Numbering of octets
within a segment is that the first data octet immediately following
the header is the lowest numbered, and the following octets are
numbered consecutively.

It is essential to remember that the actual sequence number space is
finite, though very large. This space ranges from 0 to 2**32 - 1.
Since the space is finite, all arithmetic dealing with sequence
numbers must be performed modulo 2**32. This unsigned arithmetic
preserves the relationship of sequence numbers as they cycle from
2**32 - 1 to 0 again. There are some subtleties to computer modulo
arithmetic, so great care should be taken in programming the
comparison of such values. The typical kinds of sequence number
comparisons which the TCP must perform include:

(a) Determining that an acknowledgment refers to some sequence

number sent but not yet acknowledged.

(b) Determining that all sequence numbers occupied by a segment

have been acknowledged (e.g., to remove the segment from a
retransmission queue).

(c) Determining that an incoming segment contains sequence numbers

which are expected (i.e., that the segment "overlaps" the
receive window).

A new acknowledgment (called an "acceptable ack"), is one for which
the inequality below holds:

SND.UNA < SEG.ACK =< SND.NXT

All arithmetic is modulo 2**32 and that comparisons are unsigned.
"=<" means "less than or equal".

A segment on the retransmission queue is fully acknowledged if the sum
of its sequence number and length is less than the acknowledgment
value in the incoming segment.

SEG.LEN is the number of octets occupied by the data in the segment.
It is important to note that SEG.LEN must be non-zero; segments which
do not occupy any sequence space (e.g., empty acknowledgment segments)
are never placed on the retransmission queue, so would not go through
this particular test.

Note that the acceptance test for a segment, since it requires the end
of a segment to lie in the window, is somewhat more restrictive than
is absolutely necessary. If at least the first sequence number of the
segment lies in the receive window, or if some part of the segment
lies in the receive window, then the segment might be judged
acceptable. Thus, in figure 8, at least segments 1 and 2 are
acceptable by the strict rule, and segment 3 may or may not be,
depending on the strictness of interpretation of the rule.

Note that when the receive window is zero no segments should be
acceptable except ACK segments. Thus, it should be possible for a TCP
to maintain a zero receive window while transmitting data and
receiving ACKs.

We have taken advantage of the numbering scheme to protect certain
control information as well. This is achieved by implicitly including
some control flags in the sequence space so they can be retransmitted
and acknowledged without confusion (i.e., one and only one copy of the
control will be acted upon). Control information is not physically
carried in the segment data space. Consequently, we must adopt rules
for implicitly assigning sequence numbers to control. The SYN and FIN
are the only controls requiring this protection, and these controls
are used only at connection opening and closing. For sequence number
purposes, the SYN is considered to occur before the first actual data
octet of the segment in which it occurs, while the FIN is considered
to occur after the last actual data octet in a segment in which it
occurs. The segment length includes both data and sequence space
occupying controls. When a SYN is present then SEG.SEQ is the
sequence number of the SYN.

Initial Sequence Number Selection

The protocol places no restriction on a particular connection being
used over and over again. A connection is defined by a pair of
sockets. New instances of a connection will be referred to as
incarnations of the connection. The problem that arises owing to this

January 1980
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Functional Specification
is -- "how does the TCP identify duplicate segments from previous
incarnations of the connection?" This problem becomes apparent if the
connection is being opened and closed in quick succession, or if the
connection breaks with loss of memory and is then reestablished.

To avoid confusion we must prevent segments from one incarnation of a
connection from being used while the same sequence numbers may still
be present in the network from an earlier incarnation. We want to
assure this, even if a TCP crashes and loses all knowledge of the
sequence numbers it has been using. When new connections are created,
an initial sequence number (ISN) generator is employed which selects a
new 32 bit ISN. The generator is bound to a (possibly fictitious) 32
bit clock whose low order bit is incremented roughly every 4
microseconds. Thus, the ISN cycles approximately every 4.55 hours.
Since we assume that segments will stay in the network no more than
tens of seconds or minutes, at worst, we can reasonably assume that
ISN's will be unique.

For each connection there is a send sequence number and a receive
sequence number. The initial send sequence number (ISS) is chosen by
the data sending TCP, and the initial receive sequence number (IRS) is
learned during the connection establishing procedure.

For a connection to be established or initialized, the two TCPs must
synchronize on each other's initial sequence numbers. This is done in
an exchange of connection establishing messages carrying a control bit
called "SYN" (for synchronize) and the initial sequence numbers. As a
shorthand, messages carrying the SYN bit are also called "SYNs".
Hence, the solution requires a suitable mechanism for picking an
initial sequence number and a slightly involved handshake to exchange
the ISN's. A "three way handshake" is necessary because sequence
numbers are not tied to a global clock in the network, and TCPs may
have different mechanisms for picking the ISN's. The receiver of the
first SYN has no way of knowing whether the segment was an old delayed
one or not, unless it remembers the last sequence number used on the
connection (which is not always possible), and so it must ask the
sender to verify this SYN.

The "three way handshake" and the advantages of a "clock-driven"
scheme are discussed in [4].

Knowing When to Keep Quiet

To be sure that a TCP does not create a segment that carries a
sequence number which may be duplicated by an old segment remaining in
the network, the TCP must keep quiet for a maximum segment lifetime
(MSL) before assigning any sequence numbers upon starting up or
recovering from a crash in which memory of sequence numbers in use was

January 1980
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Functional Specification

lost. For this specification the MSL is taken to be 2 minutes. This
is an engineering choice, and may be changed if experience indicates
it is desirable to do so. Note that if a TCP is reinitialized in some
sense, yet retains its memory of sequence numbers in use, then it need
not wait at all; it must only be sure to use sequence numbers larger
than those recently used.

It should be noted that this strategy does not protect against
spoofing or other replay type duplicate message problems.

3.4. Establishing a connection

The "three-way handshake" is the procedure used to establish a
connection. This procedure normally is initiated by one TCP and
responded to by another TCP. The procedure also works if two TCP
simultaneously initiate the procedure. When simultaneous attempt
occurs, the TCP receives a "SYN" segment which carries no
acknowledgment after it has sent a "SYN". Of course, the arrival of
an old duplicate "SYN" segment can potentially make it appear, to the
recipient, that a simultaneous connection initiation is in progress.
Proper use of "reset" segments can disambiguate these cases. Several
examples of connection initiation follow. Although these examples do
not show connection synchronization using data-carrying segments, this
is perfectly legitimate, so long as the receiving TCP doesn't deliver
the data to the user until it is clear the data is valid (i.e., the
data must be buffered at the receiver until the connection reaches the
ESTABLISHED state). The three-way handshake reduces the possibility
of false connections. It is the implementation of a trade-off between
memory and messages to provide information for this checking.

The simplest three-way handshake is shown in figure 9 below. The
figures should be interpreted in the following way. Each line is
numbered for reference purposes. Right arrows (-->) indicate
departure of a TCP segment from TCP A to TCP B, or arrival of a
segment at B from A. Left arrows (<--), indicate the reverse.
Ellipsis (...) indicates a segment which is still in the network
(delayed). An "XXX" indicates a segment which is lost or rejected.
Comments appear in parentheses. TCP states represent the state AFTER
the departure or arrival of the segment (whose contents are shown in
the center of each line). Segment contents are shown in abbreviated
form, with sequence number, control flags, and ACK field. Other
fields such as window, addresses, lengths, and text have been left out
in the interest of clarity.

Basic 3-Way Handshake for Connection Synchronization

Figure 9.

In line 2 of figure 9, TCP A begins by sending a SYN segment
indicating that it will use sequence numbers starting with sequence
number 100. In line 3, TCP B sends a SYN and acknowledges the SYN it
received from TCP A. Note that the acknowledgment field indicates TCP
B is now expecting to hear sequence 101, acknowledging the SYN which
occupied sequence 100.

At line 4, TCP A responds with an empty segment containing an ACK for
TCP B's SYN; and in line 5, TCP A sends some data. Note that the
sequence number of the segment in line 5 is the same as in line 4
because the ACK does not occupy sequence number space (if it did, we
would wind up ACKing ACK's!).

Simultaneous initiation is only slightly more complex, as is shown in
figure 10. Each TCP cycles from CLOSED to SYN-SENT to SYN-RECEIVED to
ESTABLISHED.

The principle reason for the three-way handshake is to prevent old
duplicate connection initiations from causing confusion. To deal with
this, a special control message, reset, has been devised. If the
receiving TCP is in a non-synchronized state (i.e., SYN-SENT,
SYN-RECEIVED), it returns to LISTEN on receiving an acceptable reset.
If the TCP is in one of the synchronized states (ESTABLISHED,
FIN-WAIT-1, FIN-WAIT-2, TIME-WAIT, CLOSE-WAIT, CLOSING), it aborts the
connection and informs its user. We discuss this latter case under
"half-open" connections below.
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Functional Specification
TCP A TCP B
1. CLOSED CLOSED
2. SYN-SENT --> <SEQ=100><CTL=SYN> ...
3. SYN-RECEIVED <-- <SEQ=300><CTL=SYN> <-- SYN-SENT
4. ... <SEQ=100><CTL=SYN> --> SYN-RECEIVED

Recovery from Old Duplicate SYN

Figure 11.

As a simple example of recovery from old duplicates, consider

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Functional Specification

figure 11. At line 3, an old duplicate SYN arrives at TCP B. TCP B
cannot tell that this is an old duplicate, so it responds normally
(line 4). TCP A detects that the ACK field is incorrect and returns a
RST (reset) with its SEQ field selected to make the segment
believable. TCP B, on receiving the RST, returns to the LISTEN state.
When the original SYN (pun intended) finally arrives at line 6, the
synchronization proceeds normally. If the SYN at line 6 had arrived
before the RST, a more complex exchange might have occurred with RST's
sent in both directions.

Half-Open Connections and Other Anomalies

An established connection is said to be "half-open" if one of the
TCPs has closed or aborted the connection at its end without the
knowledge of the other, or if the two ends of the connection have
become desynchronized owing to a crash that resulted in loss of
memory. Such connections will automatically become reset if an
attempt is made to send data in either direction. However, half-open
connections are expected to be unusual, and the recovery procedure is
mildly involved.

If at site A the connection no longer exists, then an attempt by the
user at site B to send any data on it will result in the site B TCP
receiving a reset control message. Such a message should indicate to
the site B TCP that something is wrong, and it is expected to abort
the connection.

Assume that two user processes A and B are communicating with one
another when a crash occurs causing loss of memory to A's TCP.
Depending on the operating system supporting A's TCP, it is likely
that some error recovery mechanism exists. When the TCP is up again,
A is likely to start again from the beginning or from a recovery
point. As a result, A will probably try to OPEN the connection again
or try to SEND on the connection it believes open. In the latter
case, it receives the error message "connection not open" from the
local (A's) TCP. In an attempt to establish the connection, A's TCP
will send a segment containing SYN. This scenario leads to the
example shown in figure 12. After TCP A crashes, the user attempts to
re-open the connection. TCP B, in the meantime, thinks the connection
is open.

Half-Open Connection Discovery

Figure 12.

When the SYN arrives at line 3, TCP B, being in a synchronized state,
responds with an acknowledgment indicating what sequence it next
expects to hear (ACK 100). TCP A sees that this segment does not
acknowledge anything it sent and, being unsynchronized, sends a reset
(RST) because it has detected a half-open connection. TCP B aborts at
line 5. TCP A will continue to try to establish the connection; the
problem is now reduced to the basic 3-way handshake of figure 9.

An interesting alternative case occurs when TCP A crashes and TCP B
tries to send data on what it thinks is a synchronized connection.
This is illustrated in figure 13. In this case, the data arriving at
TCP A from TCP B (line 2) is unacceptable because no such connection
exists, so TCP A sends a RST. The RST is acceptable so TCP B
processes it and aborts the connection.

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Functional Specification

TCP A TCP B

Active Side Causes Half-Open Connection Discovery

Figure 13.

In figure 14, we find the two TCPs A and B with passive connections
waiting for SYN. An old duplicate arriving at TCP B (line 2) stirs B
into action. A SYN-ACK is returned (line 3) and causes TCP A to
generate a RST (the ACK in line 3 is not acceptable). TCP B accepts
the reset and returns to its passive LISTEN state.

Figure 14.

A variety of other cases are possible, all of which are accounted for
by the following rules for RST generation and processing.

Reset Generation

As a general rule, reset (RST) should be sent whenever a segment
arrives which apparently is not intended for the current or a future
incarnation of the connection. A reset should not be sent if it is
not clear that this is the case. Thus, if any segment arrives for a
nonexistent connection, a reset should be sent. If a segment ACKs

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Functional Specification

something which has never been sent on the current connection, then
one of the following two cases applies.

1. If the connection is in any non-synchronized state (LISTEN,
SYN-SENT, SYN-RECEIVED) or if the connection does not exist, a reset
(RST) should be formed and sent for any segment that acknowledges
something not yet sent. The RST should take its SEQ field from the
ACK field of the offending segment (if the ACK control bit was set),
and its ACK bit should be reset (zero), except to refuse a initial
SYN. A reset is also sent if an incoming segment has a security level
or compartment which does not exactly match the level and compartment
requested for the connection. If the precedence of the incoming
segment is less than the precedence level requested a reset is sent.

2. If the connection is in a synchronized state (ESTABLISHED,
FIN-WAIT-1, FIN-WAIT-2, TIME-WAIT, CLOSE-WAIT, CLOSING), any
unacceptable segment should elicit only an empty acknowledgment
segment containing the current send-sequence number and an
acknowledgment indicating the next sequence number expected to be
received.

Reset Processing

All reset (RST) segments are validated by checking their SEQ-fields.
A reset is valid if its sequence number is in the window. In the case
of a RST received in response to an initial SYN any sequence number is
acceptable if the ACK field acknowledges the SYN.

The receiver of a RST first validates it, then changes state. If the
receiver was in the LISTEN state, it ignores it. If the receiver was
in SYN-RECEIVED state and had previously been in the LISTEN state,
then the receiver returns to the LISTEN state, otherwise the receiver
aborts the connection and goes to the CLOSED state. If the receiver
was in any other state, it aborts the connection and advises the user
and goes to the CLOSED state.

3.5. Closing a Connection

CLOSE is an operation meaning "I have no more data to send." The
notion of closing a full-duplex connection is subject to ambiguous
interpretation, of course, since it may not be obvious how to treat
the receiving side of the connection. We have chosen to treat CLOSE
in a simplex fashion. The user who CLOSEs may continue to RECEIVE
until he is told that the other side has CLOSED also. Thus, a program
could initiate several SENDs followed by a CLOSE, and then continue to
RECEIVE until signaled that a RECEIVE failed because the other side
has CLOSED. We assume that the TCP will signal a user, even if no
RECEIVEs are outstanding, that the other side has closed, so the user
January 1980
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Functional Specification

can terminate his side gracefully. A TCP will reliably deliver all
buffers SENT before the connection was CLOSED so a user who expects no
data in return need only wait to hear the connection was CLOSED
successfully to know that all his data was received at the destination
TCP.

There are essentially three cases:

The user initiates by telling the TCP to CLOSE the connection

The remote TCP initiates by sending a FIN control signal

Both users CLOSE simultaneously

Case 1: Local user initiates the close

In this case, a FIN segment can be constructed and placed on the
outgoing segment queue. No further SENDs from the user will be
accepted by the TCP, and it enters the FIN-WAIT-1 state. RECEIVEs
are allowed in this state. All segments preceding and including FIN
will be retransmitted until acknowledged. When the other TCP has
both acknowledged the FIN and sent a FIN of its own, the first TCP
can ACK this FIN. It should be noted that a TCP receiving a FIN
will ACK but not send its own FIN until its user has CLOSED the
connection also.

Case 2: TCP receives a FIN from the network

If an unsolicited FIN arrives from the network, the receiving TCP
can ACK it and tell the user that the connection is closing. The
user should respond with a CLOSE, upon which the TCP can send a FIN
to the other TCP. The TCP then waits until its own FIN is
acknowledged whereupon it deletes the connection. If an ACK is not
forthcoming, after a timeout the connection is aborted and the user
is told.

Case 3: both users close simultaneously

A simultaneous CLOSE by users at both ends of a connection causes
FIN segments to be exchanged. When all segments preceding the FINs
have been processed and acknowledged, each TCP can ACK the FIN it
has received. Both will, upon receiving these ACKs, delete the
connection.

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Functional Specification
TCP A TCP B
1. ESTABLISHED ESTABLISHED

top secret - 11
secret - 10
confidential - 01
unclassified - 00
The compartments are assigned by the Defense Communications Agency.
The defaults are precedence: routine, security: unclassified,
compartment: zero. A host which does not implement precedence or
security feature should clear these fields to zero for segments it
sends.

A connection attempt with mismatched security/compartment values or a
lower precedence value should be rejected by sending a reset.

Note that TCP modules which operate only at the default value of
precedence will still have to check the precedence of incoming
segments and possibly raise the precedence level they use on the
connection.

3.7. Data Communication

Once the connection is established data is communicated by the
exchange of segments. Because segments may be lost due to errors
(checksum test failure), or network congestion, TCP uses
retransmission (after a timeout) to ensure delivery of every segment.
Duplicate segments may arrive due to network or TCP retransmission.
As discussed in the section on sequence numbers the TCP performs
certain tests on the sequence and acknowledgment numbers in the
segments to verify their acceptability.

The sender of data keeps track of the next sequence number to use in
the variable SND.NXT. The receiver of data keeps track of the next

January 1980
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Functional Specification

sequence number to expect in the variable RCV.NXT. The sender of data
keeps track of the oldest unacknowledged sequence number in the
variable SND.UNA. If the data flow is momentarily idle and all data
sent has been acknowledged then the three variables will be equal.

When the sender creates a segment and transmits it the sender advances
SND.NXT. When the receiver accepts a segment it advances RCV.NXT and
sends an acknowledgment. When the data sender receives an
acknowledgment it advances SND.UNA. The extent to which the values of
these variables differ is a measure of the delay in the communication.

Normally the amount by which the variables are advanced is the length
of the data in the segment. However, when letters are used there are
special provisions for coordination the sequence numbers, the letter
boundaries, and the receive buffer boundaries.

End of Letter Sequence Number Adjustments

There is provision in TCP for the receiver of data to optionally
communicate to the sender of data on a connection at the time of the
connection synchronization the receiver's buffer size. If this is
done the receiver must use this fixed size of buffers for the lifetime
of the connection. If a buffer size is communicated then there is a
coordination between receive buffers, letters, and sequence numbers.

Each time a buffer is completed either due to being filled or due to
an end of letter, the sequence number is incremented through the end
of that buffer.

That is, whenever an EOL is transmitted, the sender advances its send
sequence number, SND.NXT, by an amount sufficient to consume all the
unused space in the receiver's buffer. The amount of space consumed
in this fashion is subtracted from the send window just as is the
space consumed by actual data.

And, whenever an EOL is received, the receiver advances its receive
sequence number, RCV.NXT, by an amount sufficient to consume all the
unused space in the receiver's buffer. The amount of space consumed
in this fashion is subtracted from the receive window just as is the
space consumed by actual data.

XXX - data octets from segment

+++ - phantom data

<----- sequence space ----->

End of Letter Adjustment

Figure 17.

In the case illustrated above, if the segment does not carry an EOL
flag, the next value of SND.NXT or RCV.NXT will be A. If it does
carry an EOL flag, the next value will be B.

The exchange of buffer size and sequencing information is done in
units of octets. If no buffer size is stated, then the buffer size is
assumed to be 1 octet. The receiver tells the sender the size of the
buffer in a SYN segment that contains the 16 bit buffer size data in
an option field in the TCP header.

Each EOL advances the sequence number (SN) to the next buffer boundary

While LBB < SEG.SEQ+SEG.LEN
Do LBB <- LBB + BS End
SN <- LBB

where LBB is the Last Buffer Beginning, and BS is the buffer size.

The CLOSE user call implies an end of letter, as does the FIN control
flag in an incoming segment.

The Communication of Urgent Information

The objective of the TCP urgent mechanism is to allow the sending user
to stimulate the receiving user to accept some urgent data and to
permit the receiving TCP to indicate to the receiving user when all
the currently known urgent data has been received by the user.

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Functional Specification

This mechanism permits a point in the data stream to be designated as
the end of "urgent" information. Whenever this point is in advance of
the receive sequence number (RCV.NXT) at the receiving TCP, that TCP
should tell the user to go into "urgent mode"; when the receive
sequence number catches up to the urgent pointer, the TCP should tell
user to go into "normal mode". If the urgent pointer is updated while
the user is in "read fast" mode, the update will be invisible to the
user.

The method employs a urgent field which is carried in all segments
transmitted. The URG control flag indicates that the urgent field is
meaningful and should be added to the segment sequence number to yield
the urgent pointer. The absence of this flag indicates that the
urgent pointer has not changed.

To send an urgent indication the user must also send at least one data
octet. If the sending user also indicates end of letter, timely
delivery of the urgent information to the destination process is
enhanced.

Managing the Window

The window sent in each segment indicates the range of sequence number
the sender of the window (the data receiver) is currently prepared to
accept. There is an assumption that this is related to the currently
available data buffer space available for this connection. The window
information is a guideline to be aimed at.

Indicating a large window encourages transmissions. If more data
arrives than can be accepted, it will be discarded. This will result
in excessive retransmissions, adding unnecessarily to the load on the
network and the TCPs. Indicating a small window may restrict the
transmission of data to the point of introducing a round trip delay
between each new segment transmitted.

The mechanisms provided allow a TCP to advertise a large window and to
subsequently advertise a much smaller window without having accepted
that much data. This, so called "shrinking the window," is strongly
discouraged. The robustness principle dictates that TCPs will not
shrink the window themselves, but will be prepared for such behavior
on the part of other TCPs.

The sending TCP must be prepared to accept and send at least one octet
of new data even if the send window is zero. The sending TCP should
regularly retransmit to the receiving TCP even when the window is
zero. Two minutes is recommended for the retransmission interval when
the window is zero. This retransmission is essential to guarantee

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Functional Specification

that when either TCP has a zero window the re-opening of the window
will be reliably reported to the other.

The sending TCP packages the data to be transmitted into segments
which fit the current window, and may repackage segments on the
retransmission queue. Such repackaging is not required, but may be
helpful.

Users must keep reading connections they close for sending until the
TCP says no more data.

In a connection with a one-way data flow, the window information will
be carried in acknowledgment segments that all have the same sequence
number so there will be no way to reorder them if they arrive out of
order. This is not a serious problem, but it will allow the window
information to be on occasion temporarily based on old reports from
the data receiver.

3.8. Interfaces

There are of course two interfaces of concern: the user/TCP interface
and the TCP/IP interface. We have a fairly elaborate model of the
user/TCP interface, but only a sketch of the interface to the lower
level protocol module.
User/TCP Interface

The functional description of user commands to the TCP is, at best,
fictional, since every operating system will have different
facilities. Consequently, we must warn readers that different TCP
implementations may have different user interfaces. However, all
TCPs must provide a certain minimum set of services to guarantee
that all TCP implementations can support the same protocol
hierarchy. This section specifies the functional interfaces
required of all TCP implementations.

TCP User Commands

The following sections functionally characterize a USER/TCP
interface. The notation used is similar to most procedure or
function calls in high level languages, but this usage is not
meant to rule out trap type service calls (e.g., SVCs, UUOs,
EMTs).

The user commands described below specify the basic functions the
TCP must perform to support interprocess communication.
Individual implementations should define their own exact format,
and may provide combinations or subsets of the basic functions in

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Functional Specification

single calls. In particular, some implementations may wish to
automatically OPEN a connection on the first SEND or RECEIVE
issued by the user for a given connection.

In providing interprocess communication facilities, the TCP must
not only accept commands, but must also return information to the
processes it serves. The latter consists of:

(b) replies to specific user commands indicating success or
various types of failure.

Open

Format: OPEN (local port, foreign socket, active/passive

[, buffer size] [, timeout] [, precedence]

[, security/compartment]) -> local connection name

We assume that the local TCP is aware of the identity of the
processes it serves and will check the authority of the process
to use the connection specified. Depending upon the
implementation of the TCP, the local network and TCP identifiers
for the source address will either be supplied by the TCP or by
the processes that serve it (e.g., the program which interfaces
the TCP network). These considerations are the result of
concern about security, to the extent that no TCP be able to
masquerade as another one, and so on. Similarly, no process can
masquerade as another without the collusion of the TCP.

If the active/passive flag is set to passive, then this is a
call to LISTEN for an incoming connection. A passive open may
have either a fully specified foreign socket to wait for a
particular connection or an unspecified foreign socket to wait
for any call. A fully specified passive call can be made active
by the subsequent execution of a SEND.

A full-duplex transmission control block (TCB) is created and
partially filled in with data from the OPEN command parameters.

On an active OPEN command, the TCP will begin the procedure to
synchronize (i.e., establish) the connection at once.

The buffer size, if present, indicates that the caller will
always receive data from the connection in that size of buffers.
This buffer size is a measure of the buffer between the user and

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Functional Specification

the local TCP. The buffer size between the two TCPs may be
different.

The timeout, if present, permits the caller to set up a timeout
for all buffers transmitted on the connection. If a buffer is
not successfully delivered to the destination within the timeout
period, the TCP will abort the connection. The present global
default is 30 seconds. The buffer retransmission rate may vary;
most likely, it will be related to the measured time for
responses from the remote TCP.

The TCP or some component of the operating system will verify
the users authority to open a connection with the specified
precedence or security/compartment. The absence of precedence
or security/compartment specification in the OPEN call indicates
the default values should be used.

TCP will accept incoming requests as matching only if the
security/compartment information is exactly the same and only if
the precedence is equal to or higher than the precedence
requested in the OPEN call.

The precedence for the connection is the higher of the values
requested in the OPEN call and received from the incoming
request, and fixed at that value for the life of the connection.

Depending on the TCP implementation, either a local connection
name will be returned to the user by the TCP, or the user will
specify this local connection name (in which case another
parameter is needed in the call). The local connection name can
then be used as a short hand term for the connection defined by
the <local socket, foreign socket> pair.

This call causes the data contained in the indicated user buffer
to be sent on the indicated connection. If the connection has
not been opened, the SEND is considered an error. Some
implementations may allow users to SEND first; in which case, an
automatic OPEN would be done. If the calling process is not
authorized to use this connection, an error is returned.

If the EOL flag is set, the data is the End Of a Letter, and the
EOL bit will be set in the last TCP segment created from the

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Functional Specification

buffer. If the EOL flag is not set, subsequent SENDs will
appear to be part of the same letter.

If the URGENT flag is set, segments resulting from this call
will have the urgent pointer set to indicate that some of the
data associated with this call is urgent. This facility, for
example, can be used to simulate "break" signals from terminals
or error or completion codes from I/O devices. The semantics of
this signal to the receiving process are unspecified. The
receiving TCP will signal the urgent condition to the receiving
process as long as the urgent pointer indicates that data
preceding the urgent pointer has not been consumed by the
receiving process. The purpose of urgent is to stimulate the
receiver to accept some urgent data and to indicate to the
receiver when all the currently known urgent data has been
received.

The number of times the sending user's TCP signals urgent will
not necessarily be equal to the number of times the receiving
user will be notified of the presence of urgent data.

If no foreign socket was specified in the OPEN, but the
connection is established (e.g., because a LISTENing connection
has become specific due to a foreign segment arriving for the
local socket), then the designated buffer is sent to the implied
foreign socket. In general, users who make use of OPEN with an
unspecified foreign socket can make use of SEND without ever
explicitly knowing the foreign socket address.

However, if a SEND is attempted before the foreign socket
becomes specified, an error will be returned. Users can use the
STATUS call to determine the status of the connection. In some
implementations the TCP may notify the user when an unspecified
socket is bound.

If a timeout is specified, then the current timeout for this
connection is changed to the new one.

In the simplest implementation, SEND would not return control to
the sending process until either the transmission was complete
or the timeout had been exceeded. However, this simple method
is both subject to deadlocks (for example, both sides of the
connection might try to do SENDs before doing any RECEIVEs) and
offers poor performance, so it is not recommended. A more
sophisticated implementation would return immediately to allow
the process to run concurrently with network I/O, and,
furthermore, to allow multiple SENDs to be in progress.

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Functional Specification

Multiple SENDs are served in first come, first served order, so
the TCP will queue those it cannot service immediately.

We have implicitly assumed an asynchronous user interface in
which a SEND later elicits some kind of SIGNAL or
pseudo-interrupt from the serving TCP. An alternative is to
return a response immediately. For instance, SENDs might return
immediate local acknowledgment, even if the segment sent had not
been acknowledged by the distant TCP. We could optimistically
assume eventual success. If we are wrong, the connection will
close anyway due to the timeout. In implementations of this
kind (synchronous), there will still be some asynchronous
signals, but these will deal with the connection itself, and not
with specific segments or letters.

NOTA BENE: In order for the process to distinguish among error
or success indications for different SENDs, it might be
appropriate for the buffer address to be returned along with the
coded response to the SEND request. TCP-to-user signals are
discussed below, indicating the information which should be
returned to the calling process.

This command allocates a receiving buffer associated with the
specified connection. If no OPEN precedes this command or the
calling process is not authorized to use this connection, an
error is returned.

In the simplest implementation, control would not return to the
calling program until either the buffer was filled, or some
error occurred, but this scheme is highly subject to deadlocks.
A more sophisticated implementation would permit several
RECEIVEs to be outstanding at once. These would be filled as,
segments arrive. This strategy permits increased throughput at
the cost of a more elaborate scheme (possibly asynchronous) to
notify the calling program that a letter has been received or a
buffer filled.

If insufficient buffer space is given to reassemble a complete
letter, the EOL flag will not be set in the response to the
RECEIVE. The buffer will be filled with as much data as it can
hold. The last buffer required to hold the letter is returned
with EOL signaled.

January 1980
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Functional Specification

The remaining parts of a partly delivered letter will be placed
in buffers as they are made available via successive RECEIVEs.
If a number of RECEIVEs are outstanding, they may be filled with
parts of a single long letter or with at most one letter each.
The return codes associated with each RECEIVE will indicate what
is contained in the buffer.

If a buffer size was given in the OPEN call, then all buffers
presented in RECEIVE calls must be of exactly that size, or an
error indication will be returned.

The URGENT flag will be set only if the receiving user has
previously been informed via a TCP-to-user signal, that urgent
data is waiting. The receiving user should thus be in
"read-fast" mode. If the URGENT flag is on, additional urgent
data remains. If the URGENT flag is off, this call to RECEIVE
has returned all the urgent data, and the user may now leave
"read-fast" mode.

To distinguish among several outstanding RECEIVEs and to take
care of the case that a letter is smaller than the buffer
supplied, the return code is accompanied by both a buffer
pointer and a byte count indicating the actual length of the
letter received.

Alternative implementations of RECEIVE might have the TCP
allocate buffer storage, or the TCP might share a ring buffer
with the user. Variations of this kind will produce obvious
variation in user interface to the TCP.

Close
Format: CLOSE(local connection name)
This command causes the connection specified to be closed. If
the connection is not open or the calling process is not
authorized to use this connection, an error is returned.
Closing connections is intended to be a graceful operation in
the sense that outstanding SENDs will be transmitted (and
retransmitted), as flow control permits, until all have been
serviced. Thus, it should be acceptable to make several SEND
calls, followed by a CLOSE, and expect all the data to be sent
to the destination. It should also be clear that users should
continue to RECEIVE on CLOSING connections, since the other side
may be trying to transmit the last of its data. Thus, CLOSE
means "I have no more to send" but does not mean "I will not
receive any more." It may happen (if the user level protocol is
not well thought out) that the closing side is unable to get rid
January 1980
Transmission Control Protocol
Functional Specification

of all its data before timing out. In this event, CLOSE turns
into ABORT, and the closing TCP gives up.

The user may CLOSE the connection at any time on his own
initiative, or in response to various prompts from the TCP
(e.g., remote close executed, transmission timeout exceeded,
destination inaccessible).

Because closing a connection requires communication with the
foreign TCP, connections may remain in the closing state for a
short time. Attempts to reopen the connection before the TCP
replies to the CLOSE command will result in error responses.

Close also implies end of letter.

Status
Format: STATUS(local connection name)

This is an implementation dependent user command and could be

excluded without adverse effect. Information returned would

typically come from the TCB associated with the connection.

This command returns a data block containing the following
information:

Depending on the state of the connection, or on the
implementation itself, some of this information may not be
available or meaningful. If the calling process is not
authorized to use this connection, an error is returned. This
prevents unauthorized processes from gaining information about a
connection.

This command causes all pending SENDs and RECEIVES to be
aborted, the TCB to be removed, and a special RESET message to
be sent to the TCP on the other side of the connection.
Depending on the implementation, users may receive abort
indications for each outstanding SEND or RECEIVE, or may simply
receive an ABORT-acknowledgment.

TCP-to-User Messages

It is assumed that the operating system environment provides a
means for the TCP to asynchronously signal the user program. When
the TCP does signal a user program, certain information is passed
to the user. Often in the specification the information will be
an error message. In other cases there will be information
relating to the completion of processing a SEND or RECEIVE or
other user call.

The TCP calls on a lower level protocol module to actually send and
receive information over a network. One case is that of the ARPA
internetwork system where the lower level module is the Internet
Protocol [2]. In most cases the following simple interface would be
adequate.

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Transmission Control Protocol
Functional Specification

The following two calls satisfy the requirements for the TCP to
internet protocol module communication:

Note that the precedence is in the TOS, and the
security/compartment is an option.

When the TCP sends a segment, it executes the SEND call supplying
all the arguments. The internet protocol module, on receiving
this call, checks the arguments and prepares and sends the
message. If the arguments are good and the segment is accepted by
the local network, the call returns successfully. If either the
arguments are bad, or the segment is not accepted by the local
network, the call returns unsuccessfully. On unsuccessful
returns, a reasonable report should be made as to the cause of the

January 1980
Transmission Control Protocol
Functional Specification

problem, but the details of such reports are up to individual
implementations.

When a segment arrives at the internet protocol module from the
local network, either there is a pending RECV call from TCP or
there is not. In the first case, the pending call is satisfied by
passing the information from the segment to the TCP. In the
second case, the TCP is notified of a pending segment.

The notification of a TCP may be via a pseudo interrupt or similar
mechanism, as appropriate in the particular operating system
environment of the implementation.

A TCP's RECV call may then either be immediately satisfied by a
pending segment, or the call may be pending until a segment
arrives.

We note that the Internet Protocol provides arguments for a type
of service and for a time to live. TCP uses the following
settings for these parameters:

Type of Service = Precedence: none, Package: stream,

Reliability: higher, Preference: speed, Speed: higher; or

00011111.

Time to Live = one minute, or 00111100.

Note that the assumed maximum segment lifetime is two minutes.
Here we explicitly ask that a segment be destroyed if it
cannot be delivered by the internet system within one minute.

January 1980
Transmission Control Protocol
Functional Specification

3.9. Event Processing

The activity of the TCP can be characterized as responding to events.
The events that occur can be cast into three categories: user calls,
arriving segments, and timeouts. This section describes the
processing the TCP does in response to each of the events. In many
cases the processing required depends on the state of the connection.

Events that occur:

User Calls

OPEN
SEND
RECEIVE
CLOSE
ABORT
STATUS

Arriving Segments
SEGMENT ARRIVES
Timeouts

USER TIMEOUT
RETRANSMISSION TIMEOUT

The model of the TCP/user interface is that user commands receive an
immediate return and possibly a delayed response via an event or
pseudo interrupt. In the following descriptions, the term "signal"
means cause a delayed response.

Error responses are given as character strings. For example, user
commands referencing connections that do not exist receive "error:
connection not open".

Please note in the following that all arithmetic on sequence numbers,
acknowledgment numbers, windows, et cetera, is modulo 2**32 the size
of the sequence number space. Also note that "=<" means less than or
equal to.

A natural way to think about processing incoming segments is to
imagine that they are first tested for proper sequence number (i.e.,
that their contents lie in the range of the expected "receive window"
in the sequence number space) and then that they are generally queued
and processed in sequence number order.

January 1980
Transmission Control Protocol
Functional Specification

When a segment overlaps other already received segments we reconstruct
the segment to contain just the new data, and adjust the header fields
to be consistent.

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Functional Specification
OPEN Call
OPEN Call
CLOSED STATE (i.e., TCB does not exist)
Create a new transmission control block (TCB) to hold connection
state information. Fill in local socket identifier, foreign
socket, precedence, security/compartment, and user timeout
information. Verify the security and precedence requested are
allowed for this user, if not return "error: precedence not
allowed" or "error: security/compartment not allowed." If active
and the foreign socket is unspecified, return "error: foreign
socket unspecified"; if active and the foreign socket is
specified, issue a SYN segment. An initial send sequence number
(ISS) is selected and the TCP receive buffer size is selected (if
applicable). A SYN segment of the form <SEQ=ISS><CTL=SYN> is sent
(this may include the buffer size option if applicable). Set
SND.UNA to ISS, SND.NXT to ISS+1, SND.LBB to ISS+1, enter SYN-SENT
state, and return.

If the caller does not have access to the local socket specified,
return "error: connection illegal for this process". If there is
no room to create a new connection, return "error: insufficient
resources".

LISTEN STATE
SYN-SENT STATE
SYN-RECEIVED STATE
ESTABLISHED STATE
FIN-WAIT-1 STATE
FIN-WAIT-2 STATE
TIME-WAIT STATE
CLOSE-WAIT STATE
CLOSING STATE

If the user should no have access to such a connection, then
return "error: connection illegal for this process".

Otherwise, return "error: connection does not exist".

LISTEN STATE

If the foreign socket is specified, then change the connection
from passive to active, select an ISS, and select the receive
buffer size. Send a SYN segment, set SND.UNA to ISS, SND.NXT to
ISS+1 and SND.LBB to ISS+1. Enter SYN-SENT state. Data
associated with SEND may be sent with SYN segment or queued for
transmission after entering ESTABLISHED state. The urgent bit if
requested in the command should be sent with the first data
segment sent as a result of this command. If there is no room to
queue the request, respond with "error: insufficient resources".
If Foreign socket was not specified, then return "error: foreign
socket unspecified".

SYN-SENT STATE

Queue for processing after the connection is ESTABLISHED.
Typically, nothing can be sent yet, anyway, because the send
window has not yet been set by the other side. If no space,
return "error: insufficient resources".

SYN-RECEIVED STATE

Queue for later processing after entering ESTABLISHED state. If
no space to queue, respond with "error: insufficient resources".

ESTABLISHED STATE

Segmentize the buffer, send or queue it for output, with a
piggybacked acknowledgment (acknowledgment value = RCV.NXT) with
the data. If there is insufficient space to remember this buffer,
simply return "error: insufficient resources".

If remote buffer size is not one octet, and, if this is the end of
a letter, do the following end-of-letter/buffer-size adjustment
processing:

If the user should no have access to such a connection, return
"error: connection illegal for this process".

Otherwise return "error: connection does not exist".

LISTEN STATE
SYN-SENT STATE
SYN-RECEIVED STATE

Queue for processing after entering ESTABLISHED state. If there
is no room to queue this request, respond with "error:
insufficient resources".

ESTABLISHED STATE

If insufficient incoming segments are queued to satisfy the
request, queue the request. If there is no queue space to
remember the RECEIVE, respond with "error: insufficient
resources".

Reassemble queued incoming segments into receive buffer and return
to user. Mark "end of letter" (EOL) if this is the case.

If RCV.UP is in advance of the data currently being passed to the
user notify the user of the presence of urgent data.

When the TCP takes responsibility for delivering data to the user
that fact must be communicated to the sender via an
acknowledgment. The formation of such an acknowledgment is
described below in the discussion of processing an incoming
segment.

FIN-WAIT-1 STATE
FIN-WAIT-2 STATE

Reassemble and return a letter, or as much as will fit, in the
user buffer. Queue the request if it cannot be serviced
immediately.

Since the remote side has already sent FIN, RECEIVEs must be
satisfied by text already reassembled, but not yet delivered to
the user. If no reassembled segment text is awaiting delivery,
the RECEIVE should get a "error: connection closing" response.
Otherwise, any remaining text can be used to satisfy the RECEIVE.

CLOSING STATE

Return "error: connection closing"

January 1980
Transmission Control Protocol
Functional Specification
CLOSE Call
CLOSE Call
CLOSED STATE (i.e., TCB does not exist)

If the user should no have access to such a connection, return
"error: connection illegal for this process".

Otherwise, return "error: connection does not exist".

LISTEN STATE

Any outstanding RECEIVEs should be returned with "error: closing"
responses. Delete TCB, return "ok".

SYN-SENT STATE

Delete the TCB and return "error: closing" responses to any
queued SENDs, or RECEIVEs.

SYN-RECEIVED STATE

Queue for processing after entering ESTABLISHED state or
segmentize and send FIN segment. If the latter, enter FIN-WAIT-1
state.

ESTABLISHED STATE

Queue this until all preceding SENDs have been segmentized, then
form a FIN segment and send it. In any case, enter FIN-WAIT-1
state.

FIN-WAIT-1 STATE
FIN-WAIT-2 STATE

Strictly speaking, this is an error and should receive a "error:
connection closing" response. An "ok" response would be
acceptable, too, as long as a second FIN is not emitted (the first
FIN may be retransmitted though).

January 1980
Transmission Control Protocol
Functional Specification
CLOSE Call
TIME-WAIT STATE

Strictly speaking, this is an error and should receive a "error:
connection closing" response. An "ok" response would be
acceptable, too. However, since the FIN has been sent and
acknowledged, nothing should be sent (or retransmitted).

CLOSE-WAIT STATE

Queue this request until all preceding SENDs have been
segmentized; then send a FIN segment, enter CLOSING state.

all data in the incoming segment is discarded. An incoming
segment containing a RST is discarded. An incoming segment not
containing a RST causes a RST to be sent in response. The
acknowledgment and sequence field values are selected to make the
reset sequence acceptable to the TCP that sent the offending
segment.

If the ACK bit is off, sequence number zero is used,

<SEQ=0><ACK=SEG.SEQ+SEG.LEN><CTL=RST,ACK>
If the ACK bit is on,
<SEQ=SEG.ACK><CTL=RST>

Return.

If the state is LISTEN then

first check for an ACK

Any acknowledgment is bad if it arrives on a connection still in
the LISTEN state. An acceptable reset segment should be formed
for any arriving ACK-bearing segment, except another RST. The
RST should be formatted as follows:

<SEQ=SEG.ACK><CTL=RST>

Return.

An incoming RST should be ignored. Return.

if there was no ACK then check for a SYN

If the SYN bit is set, check the security. If the
security/compartment on the incoming segment does not exactly
match the security/compartment in the TCB then send a reset and
return. If the SEG.PRC is less than the TCB.PRC then send a
reset and return. If the SEG.PRC is greater than the TCB.PRC
then set TCB.PRC<-SEG.PRC. Now RCV.NXT and RCV.LBB are set to
SEG.SEQ+1, IRS is set to SEG.SEQ and any other control or text
should be queued for processing later. ISS should be selected
and a SYN segment sent of the form:

SND.NXT and SND.LBB are set to ISS+1 and SND.UNA to ISS. The
connection state should be changed to SYN-RECEIVED. Note that
any other incoming control or data (combined with SYN) will be
processed in the SYN-RECEIVED state, but processing of SYN and
ACK should not be repeated. If the listen was not fully
specified (i.e., the foreign socket was not fully specified),
then the unspecified fields should be filled in now.

if there was no SYN but there was other text or control

Any other control or text-bearing segment (not containing SYN)
must have an ACK and thus would be discarded by the ACK
processing. An incoming RST segment could not be valid, since
it could not have been sent in response to anything sent by this
incarnation of the connection. So you are unlikely to get here,
but if you do, drop the segment, and return.

If the state is SYN-SENT then

first check for an ACK

If SEG.ACK =< ISS, or SEG.ACK > SND.NXT, or the
security/compartment in the segment does not exactly match the
security/compartment in the TCB, or the precedence in the
segment is less than the precedence in the TCB, send a reset

<SEQ=SEG.ACK><CTL=RST>

and discard the segment. Return.

If SND.UNA =< SEG.ACK =< SND.NXT and the security/compartment
and precedence are acceptable then the ACK is acceptable.
SND.UNA should be advanced to equal SEG.ACK, and any segments on
the retransmission queue which are thereby acknowledged should
be removed.

if the ACK is ok (or there is no ACK), check the RST bit

If the RST bit is set then signal the user "error: connection
reset", enter CLOSED state, drop the segment, delete TCB, and
return.

if the ACK is ok (or there is no ACK) and it was not a RST, check
the SYN bit
January 1980
Transmission Control Protocol
Functional Specification
SEGMENT ARRIVES

If the SYN bit is on and the security/compartment and precedence
are acceptable then, RCV.NXT and RCV.LBB are set to SEG.SEQ+1,
IRS is set to SEG.SEQ. If SND.UNA > ISS (our SYN has been
ACKed), change the connection state to ESTABLISHED, otherwise
enter SYN-RECEIVED. In any case, form an ACK segment:

<SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK>

and send it. Data or controls which were queued for
transmission may be included.

If SEG.PRC is greater than TCB.PRC set TCB.PRC<-SEG.PRC.

If there are other controls or text in the segment then continue
processing at the fifth step below where the URG bit is checked,
otherwise return.

SYN-RECEIVED STATE
ESTABLISHED STATE
FIN-WAIT-1 STATE
FIN-WAIT-2 STATE
TIME-WAIT STATE
CLOSE-WAIT STATE
CLOSING STATE

Segments are processed in sequence. Initial tests on arrival
are used to discard old duplicates, but further processing is
done in SEG.SEQ order. If a segment's contents straddle the
boundary between old and new, only the new parts should be
processed.

There are four cases for the acceptability test for an incoming
segment:

0 >0 RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND

Note that the test above guarantees that the last sequence
number used by the segment lies in the receive-window. If the
RCV.WND is zero, no segments will be acceptable, but special
allowance should be made to accept valid ACKs, URGs and RSTs.

If an incoming segment is not acceptable, an acknowledgment
should be sent in reply:

If the incoming segment is unacceptable, drop it and return.

If the security/compartment and precedence in the segment do not
exactly match the security/compartment and precedence in the TCB
then form a reset and return.

Note this check is placed following the sequence check to prevent
a segment from an old connection between these parts with a
different security or precedence from causing an abort of the
current connection.

third check the ACK field,

SYN-RECEIVED STATE

If the RST bit is off and SND.UNA < SEG.ACK =< SND.NXT then set
SND.UNA <- SEG.ACK, remove any acknowledged segments from the
retransmission queue, and enter ESTABLISHED state.

If the segment acknowledgment is not acceptable, form a reset
segment,

<SEQ=SEG.ACK><CTL=RST>

and send it, unless the incoming segment is an RST (or there is
no ACK), in which case, it should be discarded, then return.

ESTABLISHED STATE

If SND.UNA < SEG.ACK =< SND.NXT then, set SND.UNA <- SEG.ACK.
Any segments on the retransmission queue which are thereby
entirely acknowledged are removed. Users should receive
positive acknowledgments for buffers which have been SENT and
fully acknowledged (i.e., SEND buffer should be returned with
"ok" response). If the ACK is a duplicate, it can be ignored.

If the segment passes the sequence number and acknowledgment
number tests, the send window should be updated. If
SND.WL =< SEG.SEQ, set SND.WND <- SEG.WND and set
SND.WL <- SEG.SEQ.

If the remote buffer size is not one, then the
end-of-letter/buffer-size adjustment to sequence numbers may
have an effect on the next expected sequence number to be
acknowledged. It is possible that the remote TCP will
acknowledge with a SEG.ACK equal to a sequence number of an

octet that was skipped over at the end of a letter. This a mild
error on the remote TCPs part, but not cause for alarm.

FIN-WAIT-1 STATE
FIN-WAIT-2 STATE

In addition to the processing for the ESTABLISHED state, if the
retransmission queue is empty, the user's CLOSE can be
acknowledged ("ok") but do not delete the TCB.

TIME-WAIT STATE

The only thing that can arrive in this state is a retransmission
of the remote FIN. Acknowledge it, and restart the 2 MSL
timeout.

CLOSE-WAIT STATE

Do the same processing as for the ESTABLISHED state.

CLOSING STATE

If the ACK acknowledges our FIN then delete the TCB (enter the
CLOSED state), otherwise ignore the segment.

fourth check the RST bit,

SYN-RECEIVED STATE

If the RST bit is set then, if the segment has passed sequence
and acknowledgment tests, it is valid. If this connection was
initiated with a passive OPEN (i.e., came from the LISTEN
state), then return this connection to LISTEN state. The user
need not be informed. If this connection was initiated with an
active OPEN (i.e., came from SYN-SENT state) then the connection
was refused, signal the user "connection refused". In either
case, all segments on the retransmission queue should be
removed.

If the RST bit is set then, any outstanding RECEIVEs and SEND
should receive "reset" responses. All segment queues should be
flushed. Users should also receive an unsolicited general
"connection reset" signal. Enter the CLOSED state, delete the
TCB, and return.

TIME-WAIT

Enter the CLOSED state, delete the TCB, and return.

fifth, check the SYN bit,

SYN-RECEIVED
ESTABLISHED STATE

If the SYN bit is set, check the segment sequence number against
the receive window. The segment sequence number must be in the
receive window; if not, ignore the segment. If the SYN is on
and SEG.SEQ = IRS then everything is ok and no action is needed;
but if they are not equal, there is an error and a reset must be
sent.

If a reset must be sent it is formed as follows:

<SEQ=SEG.ACK><CTL=RST>

The connection must be aborted as if a RST had been received.

FIN-WAIT STATE-1
FIN-WAIT STATE-2
TIME-WAIT STATE
CLOSE-WAIT STATE
CLOSING STATE

This case should not occur, since a duplicate of the SYN which
started the current connection incarnation will have been
filtered in the SEG.SEQ processing. Other SYN's will have been
rejected by this test as well (see SYN processing for
ESTABLISHED state).

If the URG bit is set, RCV.UP <- max(RCV.UP,SEG.UP), and signal
the user that the remote side has urgent data if the urgent
pointer (RCV.UP) is in advance of the data consumed. If the
user has already been signaled (or is still in the "urgent
mode") for this continuous sequence of urgent data, do not
signal the user again.

TIME-WAIT STATE
CLOSE-WAIT STATE
CLOSING

This should not occur, since a FIN has been received from the
remote side. Ignore the URG.

seventh, process the segment text,

ESTABLISHED STATE

Once in the ESTABLISHED state, it is possible to deliver segment
text to user RECEIVE buffers. Text from segments can be moved
into buffers until either the buffer is full or the segment is
empty. If the segment empties and carries an EOL flag, then the
user is informed, when the buffer is returned, that an EOL has
been received.

If buffer size is not one octet, then do the following
end-of-letter/buffer-size adjustment processing:

This acknowledgment should be piggybacked on a segment being

transmitted if possible without incurring undue delay.

FIN-WAIT-1 STATE
FIN-WAIT-2 STATE

If there are outstanding RECEIVEs, they should be satisfied, if
possible, with the text of this segment; remaining text should
be queued for further processing. If a RECEIVE is satisfied,
the user should be notified, with "end-of-letter" (EOL) signal,
if appropriate.

TIME-WAIT STATE
CLOSE-WAIT STATE

This should not occur, since a FIN has been received from the
remote side. Ignore the segment text.

eighth, check the FIN bit,

Send an acknowledgment for the FIN. Signal the user "connection
closing", and return any pending RECEIVEs with same message. Note
that FIN implies EOL for any segment text not yet delivered to the
user. If the current state is ESTABLISHED, enter the CLOSE-WAIT
state. If the current state is FIN-WAIT-1, enter the CLOSING
state. If the current state is FIN-WAIT-2, enter the TIME-WAIT
state.

For any state if the user timeout expires, flush all queues, signal
the user "error: connection aborted due to user timeout" in general
and for any outstanding calls, delete the TCB, and return.

RETRANSMISSION TIMEOUT

For any state if the retransmission timeout expires on a segment in
the retransmission queue, send the segment at the front of the
retransmission queue again, reinitialize the retransmission timer,
and return.

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GLOSSARY

BBN Report 1822, "The Specification of the Interconnection of
a Host and an IMP". The specification of interface between a
host and the ARPANET.

ACK

A control bit (acknowledge) occupying no sequence space, which
indicates that the acknowledgment field of this segment
specifies the next sequence number the sender of this segment
is expecting to receive, hence acknowledging receipt of all
previous sequence numbers.

ARPANET message

The unit of transmission between a host and an IMP in the
ARPANET. The maximum size is about 1012 octets (8096 bits).

ARPANET packet

A unit of transmission used internally in the ARPANET between
IMPs. The maximum size is about 126 octets (1008 bits).

buffer size

An option (buffer size) used to state the receive data buffer
size of the sender of this option. May only be sent in a
segment that also carries a SYN.

connection

A logical communication path identified by a pair of sockets.

datagram

A message sent in a packet switched computer communications
network.

Destination Address

The destination address, usually the network and host
identifiers.

EOL

A control bit (End of Letter) occupying no sequence space,
indicating that this segment ends a logical letter with the
last data octet in the segment. If this end of letter causes
a less than full buffer to be released to the user and the
connection buffer size is not one octet then the
end-of-letter/buffer-size adjustment to the receive sequence
number must be made.

January 1980
Transmission Control Protocol
Glossary

FIN

A control bit (finis) occupying one sequence number, which
indicates that the sender will send no more data or control
occupying sequence space.

fragment

A portion of a logical unit of data, in particular an internet
fragment is a portion of an internet datagram.

FTP

A file transfer protocol.

header

Control information at the beginning of a message, segment,
fragment, packet or block of data.

host

A computer. In particular a source or destination of messages
from the point of view of the communication network.

Identification

An Internet Protocol field. This identifying value assigned
by the sender aids in assembling the fragments of a datagram.

IMP

The Interface Message Processor, the packet switch of the
ARPANET.

internet address

A source or destination address specific to the host level.

internet datagram

The unit of data exchanged between an internet module and the
higher level protocol together with the internet header.

internet fragment

A portion of the data of an internet datagram with an internet
header.

IP

Internet Protocol.

IRS

The Initial Receive Sequence number. The first sequence
number used by the sender on a connection.

January 1980
Transmission Control Protocol
Glossary

ISN

The Initial Sequence Number. The first sequence number used
on a connection, (either ISS or IRS). Selected on a clock
based procedure.

ISS

The Initial Send Sequence number. The first sequence number
used by the sender on a connection.

leader

Control information at the beginning of a message or block of
data. In particular, in the ARPANET, the control information
on an ARPANET message at the host-IMP interface.

left sequence

This is the next sequence number to be acknowledged by the
data receiving TCP (or the lowest currently unacknowledged
sequence number) and is sometimes referred to as the left edge
of the send window.

letter

A logical unit of data, in particular the logical unit of data
transmitted between processes via TCP.

local packet

The unit of transmission within a local network.

module

An implementation, usually in software, of a protocol or other
procedure.

MSL

Maximum Segment Lifetime, the time a TCP segment can exist in
the internetwork system. Arbitrarily defined to be 2 minutes.

octet

An eight bit byte.

Options

An Option field may contain several options, and each option
may be several octets in length. The options are used
primarily in testing situations; for example, to carry
timestamps. Both the Internet Protocol and TCP provide for
options fields.

packet

A package of data with a header which may or may not be

January 1980
Transmission Control Protocol
Glossary

logically complete. More often a physical packaging than a
logical packaging of data.

port

The portion of a socket that specifies which logical input or
output channel of a process is associated with the data.

process

A program in execution. A source or destination of data from
the point of view of the TCP or other host-to-host protocol.

PSN

A Packet Switched Network. For example, the ARPANET.

RCV.BS

receive buffer size, the remote buffer size

RCV.LBB

receive last buffer beginning

RCV.NXT

receive next sequence number

RCV.UP

receive urgent pointer

RCV.WND

receive window

receive last buffer beginning

This is the sequence number of the first octet of the most
recent buffer. This value is use in calculating the next
sequence number when a segment contains an end of letter
indication.

receive next sequence number

This is the next sequence number the local TCP is expecting to
receive.

receive window

This represents the sequence numbers the local (receiving) TCP
is willing to receive. Thus, the local TCP considers that
segments overlapping the range RCV.NXT to
RCV.NXT + RCV.WND - 1 carry acceptable data or control.
Segments containing sequence numbers entirely outside of this
range are considered duplicates and discarded.

January 1980
Transmission Control Protocol
Glossary

RST

A control bit (reset), occupying no sequence space, indicating
that the receiver should delete the connection without further
interaction. The receiver can determine, based on the
sequence number and acknowledgment fields of the incoming
segment, whether it should honor the reset command or ignore
it. In no case does receipt of a segment containing RST give
rise to a RST in response.

RTP

Real Time Protocol: A host-to-host protocol for communication
of time critical information.

Rubber EOL

An end of letter (EOL) requiring a sequence number adjustment
to align the beginning of the next letter on a buffer
boundary.

SEG.ACK

segment acknowledgment

SEG.LEN

segment length

SEG.PRC

segment precedence value

SEG.SEQ

segment sequence

SEG.UP

segment urgent pointer field

SEG.WND

segment window field

segment

A logical unit of data, in particular a TCP segment is the
unit of data transfered between a pair of TCP modules.

segment acknowledgment

The sequence number in the acknowledgment field of the
arriving segment.

segment length

The amount of sequence number space occupied by a segment,
including any controls which occupy sequence space.

January 1980
Transmission Control Protocol
Glossary

segment sequence

The number in the sequence field of the arriving segment.

send last buffer beginning

This is the sequence number of the first octet of the most
recent buffer. This value is used in calculating the next
sequence number when a segment contains an end of letter
indication.

send sequence

This is the next sequence number the local (sending) TCP will
use on the connection. It is initially selected from an
initial sequence number curve (ISN) and is incremented for
each octet of data or sequenced control transmitted.

send window

This represents the sequence numbers which the remote
(receiving) TCP is willing to receive. It is the value of the
window field specified in segments from the remote (data
receiving) TCP. The range of sequence numbers which may be
emitted by a TCP lies between SND.NXT and
SND.UNA + SND.WND - 1.

SND.BS

send buffer size, the local buffer size

SND.LBB

send last buffer beginning

SND.NXT

send sequence

SND.UNA

left sequence

SND.UP

send urgent pointer

SND.WL

send sequence number at last window update

SND.WND

send window

socket

An address which specifically includes a port identifier, that
is, the concatenation of an Internet Address with a TCP port.

January 1980
Transmission Control Protocol
Glossary

Source Address

The source address, usually the network and host identifiers.

SYN

A control bit in the incoming segment, occupying one sequence
number, used at the initiation of a connection, to indicate
where the sequence numbering will start.

TCB

Transmission control block, the data structure that records
the state of a connection.

TCB.PRC

The precedence of the connection.

TCP

Transmission Control Protocol: A host-to-host protocol for
reliable communication in internetwork environments.

TOS

Type of Service, an Internet Protocol field.

Type of Service

An Internet Protocol field which indicates the type of service
for this internet fragment.

URG

A control bit (urgent), occupying no sequence space, used to
indicate that the receiving user should be notified to do
urgent processing as long as there is data to be consumed with
sequence numbers less than the value indicated in the urgent
pointer.

urgent pointer

A control field meaningful only when the URG bit is on. This
field communicates the value of the urgent pointer which
indicates the data octet associated with the sending user's
urgent call.